2,497 research outputs found
A de Finetti representation theorem for infinite dimensional quantum systems and applications to quantum cryptography
According to the quantum de Finetti theorem, if the state of an N-partite
system is invariant under permutations of the subsystems then it can be
approximated by a state where almost all subsystems are identical copies of
each other, provided N is sufficiently large compared to the dimension of the
subsystems. The de Finetti theorem has various applications in physics and
information theory, where it is for instance used to prove the security of
quantum cryptographic schemes. Here, we extend de Finetti's theorem, showing
that the approximation also holds for infinite dimensional systems, as long as
the state satisfies certain experimentally verifiable conditions. This is
relevant for applications such as quantum key distribution (QKD), where it is
often hard - or even impossible - to bound the dimension of the information
carriers (which may be corrupted by an adversary). In particular, our result
can be applied to prove the security of QKD based on weak coherent states or
Gaussian states against general attacks.Comment: 11 pages, LaTe
Itinerant in-plane magnetic fluctuations and many-body correlations in NaCoO
Based on the {\it ab-initio} band structure for NaCoO we derive the
single-electron energies and the effective tight-binding description for the
bands using projection procedure. Due to the presence of the
next-nearest-neighbor hoppings a local minimum in the electronic dispersion
close to the point of the first Brillouin zone forms. Correspondingly,
in addition to a large Fermi surface an electron pocket close to the
point emerges at high doping concentrations. The latter yields the new
scattering channel resulting in a peak structure of the itinerant magnetic
susceptibility at small momenta. This indicates dominant itinerant in-plane
ferromagnetic fluctuations above certain critical concentration , in
agreement with neutron scattering data. Below the magnetic susceptibility
shows a tendency towards the antiferromagnetic fluctuations. We further analyze
the many-body effects on the electronic and magnetic excitations using various
approximations applicable for different ratio.Comment: 10 page
Relation between Vortex core charge and Vortex Bound States
Spatially inhomogeneous electron distribution around a single vortex is
discussed on the basis of the Bogoliubov-de Gennes theory. The spatial
structure and temperature dependence of the electron density around the vortex
are presented. A relation between the vortex core charge and the vortex bound
states (or the Caroli-de Gennes-Matricon states) is pointed out. Using the
scanning tunneling microscope, information on the vortex core charge can be
extracted through this relation.Comment: 5 pages, 3 figures; minor changes; Version to appear in JPSJ 67,
No.10, 199
Determination of the Coherence Length and the Cooper-Pair Size in Unconventional Superconductors by Tunnelling Spectroscopy
The main purpose of the paper is to discuss a possibility of the
determination of the values of the coherence length and the Cooper-pair size in
unconventional superconductors by using tunnelling spectroscopy. In the mixed
state of type-II superconductors, an applied magnetic field penetrates the
superconductor in the form of vortices which form a regular lattice. In
unconventional superconductors, the inner structure of a vortex core has a
complex structure which is determined by the order parameter of the
superconducting state and by the pairing wavefunction of the Cooper pairs. In
clean superconductors, the spatial variations of the order parameter and the
pairing wavefunction occur over the distances of the order of the coherence
length and the Cooper-pair size, respectively. Therefore, by performing
tunnelling spectroscopy along a line passing through a vortex core, one is
able, in principle, to estimate the values of the coherent length and the
Cooper-pair size.Comment: 13 pages, including 17 figure
The Structure of a Vortex in the t-J Model
We study the single-vortex solution of the t-J model within
resonating-valence-bond (RVB) mean-field theory. We find two types of vortex
cores, insulating and metallic, depending on the parameters of the model. The
pairing order parameter near both cores have
symmetry. For some range of the calculated tunneling spectrum of the
metallic vortex core agrees qualitatively with the STM tunneling data for
BSCCO
Theory of vortex lattice effects on STM spectra in d-wave superconductors
Theory of scanning tunneling spectroscopy of low energy quasiparticle (QP)
states in vortex lattices of d-wave superconductors is developed taking account
of the effects caused by an extremely large extension of QP wavefunctions in
the nodal directions and the band structure in the QP spectrum. The oscillatory
structures in STM spectra, which correspond to van Hove singularities are
analysed. Theoretical calculations carried out for finite temperatures and
scattering rates are compared with recent experimental data for high
temperature cuprates.Comment: 4 pages, 3 eps figures, M2S-HTSC-VI conference paper, using Elsevier
style espcrc2.st
Kohn-Luttinger instability of the t-t' Hubbard model in two dimensions: variational approach
An effective Hamiltonian for the Kohn-Luttinger superconductor is constructed
and solved in the BCS approximation. The method is applied to the t-t' Hubbard
model in two dimensions with the following results: (i) The superconducting
phase diagram at half filling is shown to provide a weak-coupling analog of the
recently proposed spin liquid state in the J_1-J_2 Heisenberg model. (ii) In
the parameter region relevant for the cuprates we have found a nontrivial
energy dependence of the gap function in the dominant d-wave pairing sector.
The hot spot effect in the angular dependence of the superconducting gap is
shown to be quite weak
Strong diamagnetic response and specific heat anomaly above T_c in underdoped La_(2-x)Sr_xCuO_4
By measuring AC susceptibility using a very low amplitude of the AC field (<1
mG) it is shown that underdoped samples of La_(2-x)Sr_xCuO_4 (LASCO), are
diamagnetic in a temperature region above T_c up to a temperature T^*. This
behavior is only observed with AC fields along the c-axis whereas for fields in
the ab-plane no diamagnetism above Tc was detected. The diamagnetism is almost
frequency independent in the frequency range 0.1-10 kHz. At T* a broad step
anomaly in the specific heat is inferred through measurements of the elastic
constant c33. We suggest that the observed diamagnetism and the anomaly in the
elastic constant are associated with the existence of phase incoherent Cooper
pairs between Tc and T*.Comment: 5 pages 7 figures, to appear in Phys. rev
Instability of a Landau Fermi liquid as the Mott insulator is approached
We examine a two-dimensional Fermi liquid with a Fermi surface which touches
the Umklapp surface first at the 4 points as the
electron density is increased. Umklapp processes at the 4 patches near lead the renormalization group equations to scale to strong
coupling resembling the behavior of a 2-leg ladder at half-filling. The
incompressible character of the fixed point causes a breakdown of Landau theory
at these patches. A further increase in density spreads the incompressible
regions so that the open Fermi surface shrinks to 4 disconnected segments. This
non-Landau state, in which parts of the Fermi surface are truncated to form an
insulating spin liquid, has many features in common with phenomenological
models recently proposed for the cuprate superconductors.Comment: Minor changes. LaTeX2e, 12 pages, 5 figures. J. Phys. CM 10 (1998)
L38
Truncation of a 2-dimensional Fermi surface due to quasiparticle gap formation at the saddle points
We study a two-dimensional Fermi liquid with a Fermi surface containing the
saddle points and . Including Cooper and Peierls channel
contributions leads to a one-loop renormalization group flow to strong coupling
for short range repulsive interactions. In a certain parameter range the
characteristics of the fixed point, opening of a spin and charge gap and
dominant pairing correlations are similar to those of a 2-leg ladder at
half-filling. An increase of the electron density we argue leads to a
truncation of the Fermi surface with only 4 disconnected arcs remaining.Comment: Reference added. RevTeX 4 pages incl. 4 ps file
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